Many processes for bonding of hydrophilic surfaces to free radical curing thermoset resins are known, for instance copper conductors to rubber insulation, but modern industries need further improvement in adhesion. To follow the copper/rubber example, the wire and cable industry needs further improvements in such adhesion. One known method for adhesion between copper and rubber is the use of resins or other thermoplastics to produce a tie coat between the insulation and the copper. It is also known to treat copper surfaces with methylacrylato-chromium complexes (e.g., chromium, aqua chloro hydroxy methacrylate complexes) before adhesion to rubber.
In the practice of prior art processes less than the theoretical maximum bond strength is developed. One cause for this deficiency is that wetting of a hydrophilic surface, for instance, a metal surface, with the chromium complex solution can be incomplete and the tensile bond strength between the hydrophilic surfaces and thermoset resin therefore is less than desired for certain applications. While the wetting of surfaces can readily be improved by the use of a surfactant either as a prewash or as a component of the chromium complex solution, when sufficient surfactant is present to effect more complete wetting, surfactants are found to interfere with the bonding action of the chromium complex, probably by altering the surface polarity, causing disorientation of the chromium complex on the hydrophilic surface. Surfactants are thus relatively ineffective in this use.
It is desirable to have a process which provides hydrophilic surface to thermoset resin tensile bond strengths several times higher than achieved using known processes. An improved process is applicable in the wire and cable industry, as well as other industries, and is provided by the present invention.